Sound recording



April J. H. MCLEQD El AL 2,196,735

SOUND RECORDING Filed Oct. 13, 1937 John hf McLeod INVENTORS Mil-Wlmxbwz- ATTORNEYS Patented Apr. 9, 1940 PATENT "OFFICE.

SOUND RECORDING John H. McLeod and Walter K. Grimwood,

Rochester, N. Y., assignors to Eastman Kodak Company, Rochester, N. Y.,a corporation of New Jersey Application ctober 13, 1937, Serial N6.168,779

3 Claims.

This invention relates to sound recording and particularly to push-pullsound-on-film record- It is an object of the invention to provide aso-called noiseless or class B push-pull recording system using variablearea.

t is a particular object to provide a toed-in variable area class Bpush-pull sound record and a method of producing such a record.

It is a further object of the invention to provide a ribbon type lightvalve of extremely simple construction capable of producing theabovementioned sound record.

According to the invention, a ribbon type light valve consisting of asingle ribbon in the usual magnetic field and an aperture at least aswide as the ribbon, is connected to an'input circuit carrying the signalto be recorded. An optical system including a source of light andsuitable spherical and cylindrical lenses projects a shadow image ofthis ribbon to a sensitive film on which the record is to be made. Theoptical system must besuch thatthe edges of this shadow image are sharp(i. e., the image must be longitudinally sharp) and the image shouldexactly fill the sound track area for zero input signal. Furthermore, a

mask exactly overshadowing the remainder of the film is placed as nearlyas possible in the plane of the ribbon or of a real image thereof, suchas at the film itself or at any intermediate image. Positioning the masknear an intermediate image is preferable since otherwise, the mask wouldtend to interfere with the vibration of the ribbon or the motion of thefilm when placed near enough to either to insure exact and sharpmasking.

For zero input signal, no light reaches the film since the shadows ofthe ribbon and that of the mask are exactly supplementary to each other.

On the introduction of a modulation, the positive duced from the twosides in accordance with the,

negative and positive amplitudes respectively.

The usual advantages of class B push-pull recording as discussed in acopending application, Serial Number 168,777, patented June 6, 1939, No.2,161,369, filed concurrently herewith by McLeod, one of the presentinventors, and

Morse, are thus obtained in a unique and convenient manner requiringrelatively simple light valves and in the preferable embodiment, asimple uni-axial optical system. The positive sound track printed fromthe above negative sound track consists of a succession of transparentareashavin'g widths varying in accordance with the input signal andalternately, according to the positive and negative halves of eachcycle, bordering on the sides of the area assigned to the sound record.As is the case with all class B push pullrecordings, the positive soundtrack approaches a maximum density for small input signal. 1 i

Other objects and advantages of the invention will be apparent'from thefollowing description when read inconnection with the accompanyingdrawing, in which: 7

Fig. 1 is a schematic drawing of a sound recording system incorporatingthe invention. Fig.2 is aplan of the optical system employed in.Fig.l.,v c

Fig. 3 schematically shows a recordingsystem incorporating a preferredembodiment of the invention.

Fig. 4 is a plan of the optical system employed in Fig. 3.

Fig. 5 is an elevation of this optical system.

In Fig. 1 an input signal picked up by a microphone I0 is passed throughits accompanying amplifier H to any suitable input circuit shown as atransformer having a primary l2 and a secondary [3. A ribbon type lightvalve comprising a ribbon M in the usual magnetic field (not shown inFig. 1 but portions it of the pole pieces appear in Fig. 2) is connectedacross the input circuit. An optical system comprising a source of light20. spherical lenses 2| and 23, and a cylindrical lens 26 projects asharp shadow image of the ribbon l4 onto a film 30 having the usualpicture area 3|. Any suitable optical system may be used provided theshadow image of the ribbon l4 exactly fills the area 32 providedtherefore on the film. That is, the edges of the ribbon 14 must beexactly imaged by the lens system shown as the spherical lens 23 and thecylindrical lens 26 along the lines 33 and 34. This is clearly shown inFig. 2.

The height of this shadow image is limited by a mask 24 having a slit25, preferably positioned near the lens 23 and arranged so that thecylindrical lens .26 projects a sharp image of this slit 25 onto thefilm. This is the usual expedient for providing sharp resolution.

According to the invention, a mask 22, shown approximately in the planeof the ribbon I 4 is positioned to exactly overshadow the remainder ofthe film not covered by the shadow image of the ribbon l4 itself. Asdiscussed in the general case above, this mask 22 may be in the plane ofthe ribbon hi, the plane of the film or in the plane of any intermediateimage of this ribbon l 5.

The preferred embodiment of the invention wherein the mask is in theplane of an intermediate image is shown in Fig. 3, wherein a lens systemshown as a simple positive lens 21 projects an aerial image of theribbon M at the position indicated by the broken lines 14. Themask 22 isreplaced by a mask 22' which together with the shadow image Hi just cutsout all the light when no input signal is present.

Figs. 4 and Bindicate clearly how the lens 21 forms an exact image M ofthe ribbon I4 in the plane of the mask 22 and further how this image isprojected by the lens 23 onto the film 30. How the cylindrical lens 26and slit 25 perform their function of exactly determining the height ofthe recording beam as it strikes the film 30 is also clearly shown.

In Figs. 1 and 3, the single aperture in the magnet of the light valveis indicated by broken line Hi. In Figs. 2, 4, and 5, this aperture isshown by indicating the magnet I 6 in cross section. The mounting andtensioning of a ribbon in the magnetic field is well known in the artand need not be discussed in detail here. The aperture l5 must be atleast as wide as the ribbon 4. In fact, it is conceivable that it couldbe arranged to be exactly the same width and to act as the mask for thisribbon H, but from a practical point of view it is not convenient to doso and hence the only arrangements shown include the introduction of anextra mask 22 or 22.

Having thus described our invention and the manner in which it may bepracticed, we wish to point out that it is not limited to the specificembodiments shown, but is of the scope of the appended claims.

recording system comprising an input circuit carrying the signal to berecorded, a single ribbon, single aperture light valve across the inputcircuit, a sensitive film having a portion adapted to receive the soundrecord and an optical system including a source of light, for projectinga longitudinally sharp shadow image of the ribbon onto the film, saidimage exactly covering said portion for zero input signal and stationarymasking means in a plane other than that of the ribbon exactlyovershadowing the remainder of the film.

2. A variable area push-pull sound-on-film recording system comprisingan input circuit carrying the signal to be recorded, a single ribbonlight valve having an aperture at least as wide as the ribbon which isconnected across the input circuit and an optical system including asource of light for projecting a longitudinally sharp shadow image ofthe ribbon in the plane adapted to be occupied by the sensitiverecording medium and stationary masking means for limiting the width ofsaid plane to exactly the width of said shadow image, whereby, at zeroinput signal said shadow image and the shadow of said masking means areexactly supplementary.

' 3. A variable area push-pull sound-on-film recording system comprisingan input circuit for carrying the signal to be recorded, a single ribbonsingle aperture light valve across the input circuit and an opticalsystem including a sourceof light for illuminating the valve, lenses forprojecting a longitudinally sharp intermediate shadow image of theribbon between the valve and the plane for the film and for reimagingthis shadow image in the film plane and stationary masking means in theplane of the intermediate image exactly supplementing its width at zeroinput signal only.

JOHN H. McLEOD. WALTER. K. GRIMWOOD.

